In the operation of a mortar, a mortar round is dropped into the mortar barrel where it falls toward the barrel's bottom and strikes the firing pin. The round's propelling charge is then ignited and the resulting propelling gasses expel the round out of the barrel.
In order for the round to be easily inserted and for it to proceed unimpeded to the bottom of the barrel, there has to be a sufficient clearance between the round and the barrel's inner wall. However, when the propelling charge is ignited and propelling gases generated, a gas seal has to be formed between the round and the barrel in order for the gasses to expel the round from the barrel.
To provide such a seal, the round contains an ‘obturator’ (sealing) ring positioned in its widest section. This ring customarily has a split design which, while not impeding the round's travel down the barrel, enables the ring to expand under pressure of the propelling gasses and provide a seal between the round and the barrel.
The present obturators rings, however, do not provide adequate seals, and some propelling gasses escape, thus reducing the energy and the resulting range of the round. Also, the present design of the obturator rings prevents their use with rifled mortar barrels. Without a rifled barrel the round does not spin and its accuracy is diminished as a result.
There is a rifled mortar system made by TDA Armements SAS of France (‘TDA’) which has its rounds containing a special obturator ring with splines matching the rifling of the barrel. However, the system requires a careful round insertion with aligning of the round's splines to the barrel rifling. This presents a difficulty in high-stress combat environments and requires high degree of cleanliness of the barrel as well as the round's ring, lest the round be jammed on its way down the barrel. Still, by necessity, there has to be sufficient clearance between the ring's and barrel features to enable the round's travel down the barrel. These clearances create channels for the propelling gas' escape. So, while greatly improving the round's accuracy by imparting a spin to it, the system still has shortcomings in terms of ease and reliability of operation and reduction of the potential range of its rounds.
The rifled barrel working with the splined ring has been limited only to one larger caliber 155 mm system by TDA. The adoption of this system for smaller caliber mortars has not happened, some of the reasons being that smaller caliber systems would be more susceptible to fouling and difficulties inserting the round since their rifling would be proportionately smaller and their rounds lighter than the larger caliber's to overcome any friction they may encounter while traveling down the barrel.